In the neighbourhood of Tame Monsters

A study of galaxies near low-redshift quasars

Department of Physics and Astronomy, Uppsala Universitet, Box 516, 751 20 Uppsala, Sweden
e-mail: beatriz.villarroel@physics.uu.se

Received: 22 November 2011
Accepted: 23 April 2012

Abstract

Context. The impact of quasars on their galaxy neighbours is an important factor in the understanding of galaxy evolution models.

Aims. The aim of this work is to characterize the intermediate-scale environments of quasars at low redshift (z < 0.2) with the most statistically complete sample to date using the seventh data release of the Sloan Digital Sky Survey.

Methods. We have used 305 quasar-galaxy associations with spectroscopically measured redshifts within the projected distance range of 350 kpc, to calculate how surface densities of galaxies, colors, degree of ionization, dust extinction and star-formation rates change as a function of the distance to our quasar sample. We also identify the companion active galactic nuclei from our main galaxy sample and calculate surface density for different galaxy types. We have done this in three different quasar-galaxy redshift difference ranges  |Δz|  < 0.001, 0.006, and 0.012.

Results. Our results suggest that there is a significant increase of the surface density of blue neighbours around our low-redshift quasar sample that is steeper than around non-active field galaxies of the same luminosity and redshift range. This may indicate that quasar formation is accomplished via a merging scenario. No significant changes in star formation rate, dust extinction, degree of ionization or color as a function of distance from the quasars was observed. We could not observe any direct effects from quasars on the their companion galaxies.